The present invention relates to polymerizing ethylene at high pressure and high temperature and more particularly to the widening of the polyethylene molecular weight distribution by using two reactors and two separators.
It is known to polymerize ethylene at high pressure and temperature using a Ziegler-type catalytic system comprising on one hand at least one halogenated compound of a transition metal and on the other hand at least one activator selected from the hydrides and the organo-metallic compounds of the metals of groups I through III of the periodic table. However, the manufacture by this process of high-density polyethylene or of ethylene copolymers with a fluidity index (measured by the ASTM D 1238-73 standard) between approximately 0.1 and 2 and a polydispersity index (Mw/Mn ratio of the average molecular mass in weight to the average molecular mass as a number) exceeding 8 is especially delicate. In particular, the selection of the catalytic system, the selection of the temperature and pressure conditions, and the use of a reactor comprising several zones operating under different conditions do not suffice to modify the polydispersity index in the desired direction.
One solution to the above cited problem is described in French Patent No. 2,346,374, consisting in partially liquefying at a temperature between -50.degree. C. and +20.degree. C. the gaseous phase from a first reactor at a pressure of 80 to 250 bars, in expanding to a pressure between 10 and 60 bars, and in then separating this flow in a second separator, the liquid phase being recompressed and moved to a first zone of the reactor while the gaseous phase is recompressed and moved to a second zone of the reactor. This solution is industrially satisfactory because it achieves a polydispersity index of about 15, but it suffers from the drawback of being costly in energy.